The history concerning foam molding is old; a technique of obtaining a foamed product made of a resin by injection molding is disclosed in, for example, U.S. Pat. No. 3,268,639 and U.S. Pat. No. 3,384,691. Recently, foam molding methods using chemical foaming agents and physical foaming agents can be learnt from textbooks for synthetic resin molding and the like. As is known from styrofoam and the like, conventional foam molding exhibits a very high foaming magnification, and hence has an advantage in terms of weight reduction but suffers low mechanical strength. In addition, many problems are posed in the use of chemical foaming agents in terms of toxicity, mold corrosion, molding environment degradation, difficulty in handling, and the like. In contrast to this, a foam is disclosed in U.S. Pat. No. 3,796,779, which is obtained by directly blowing an inert gas, e.g., carbon dioxide gas, into a molten resin material, letting the gas permeate the resin, and then cooling the resin. The use of an inert gas as a foaming agent solves the problems of toxicity, mold corrosion, molding environment degradation, difficulty in handling, and the like. According to U.S. Pat. No. 3,796,779, however, since the gas is directly blown into the molten resin, the resin and gas are not uniformly mixed to form island structures in various cell shapes. For example, the resultant structure partly decreases in strength. That is, it is very difficult to control a foamed state. To eliminate these drawbacks, a method of molding very small foams called microcells was invented in Massachusetts Institute of Technology in the U.S. early in the 1980s. This method and apparatus are disclosed in U.S. Pat. No. 4,473,665, U.S. Pat. No. 5,158,986, U.S. Pat. No. 5,160,674, U.S. Pat. No. 5,334,356, U.S. Pat. No. 5,571,848, and U.S. Pat. No. 5,866,053. According to the method and apparatus developed Massachusetts Institute of Technology, a supercritical inert gas is blown into a portion of the plasticizing unit of an injection molding machine in which a resin is molten, and the molten resin and gas are sufficiently mixed by a static mixer, while pressure and temperature control is performed. In the resultant foamed product, many small cells with sizes of 25 μm or less are uniformly dispersed. Since the cell diameters are small, a molded product that is almost free from a decrease in strength can be obtained. In addition, the following method is known. When a resin is to be injected into a mold, the mold is filled with a gas and pressurized with atmospheric pressure or more in advance, and the resin is injected. After the resin is completely charged into the mold, the gas pressure is released to cause gas foaming in the resin.
In the conventional method, since a gas is directly blown into a molten resin material, a molten resin portion that comes into contact with the gas is rapidly cooled when the gas is blown. If the gas is continuously blown into the resin, the molten resin is mostly cooled. As a consequence, the viscosity of the resin increases, and it takes much time to restore the resin temperature and viscosity that are suited to molding. In addition, when a gas is to be heated to a temperature near the melting temperature of the resin in advance, the volume of the gas increases with a rise in temperature. If, therefore, the gas is directly blown into the molten resin, since the gas pressure in the resin is low, the foaming magnification after the charge of the resin into the mold is excessively low. A method of compensating for this drawback is available, in which the temperature and pressure of a gas are raised to maintain the gas concentration, and the gas is then blown into a molten resin. In this case, however, the pressure of the gas is very high, and hence flows into a molten resin as soon as it is blown thereinto. This makes it difficult to control the amount of gas blown. In addition, since the gas is quickly blown into the molten resin, the resin into which the gas is blown is formed into a two-isolated-layer structure. To uniformly disperse the gas into the resin, the gas and resin must be repeatedly kneaded mechanically by a static mixer or the like. This complicates the apparatus and prolongs the cycle, resulting in a deterioration in productivity. The plasticizing unit of an injection molding machine or extrusion machine is configured to apply a certain pressure to a molten resin to purge air from a material or a material during metering. If, therefore, a gas is blown into a molten resin and metered as in the prior art, the gas is discharged to the metering portion side of the plasticizing unit before the blown gas completely dissolves in the resin. According to the method of filling a mold with a gas, applying a pressure equal to atmospheric pressure or more in injecting a resin into a mold, and then releasing the gas pressure after the resin is charged, if the filling speed is high, the pressure of the gas charged into the mold cannot be controlled. As a result, the charge gas causes a short shot. If the filling speed of a resin is decreased, the gas charged into the mold can be controlled, and the pressure of the gas during and after the charge of a resin can be controlled. Owing to the low filling speed, however, a large skin layer is formed, which solidifies when it comes into contact with the mold as the mold is cooled. For this reason, in the foaming distribution of the molded product, the foaming differences between the surface and the central portion in the direction of thickness and between a portion near the gate and the finally filled portion become vary large.
A technique associated with an audio equipment structure is disclosed in, for example, Japanese Patent Publication No. 60-47796.
The technique disclosed in this reference is associated with a structural member having a three-layer structure formed by a surface layer and core layer, which is formed by injection molding.
The technique disclosed in this reference is a technique of taking measures against the mechanical and audio vibration sources of audio equipment or vibrations from other sources.
As a technique of preventing resonance, a technique using a sandwich structure is disclosed in Japanese Patent Publication No. 52-28657.
When a resin molded product molded by a resin material is to be used as the above audio equipment or the housing structural member of video equipment, a foam molding technique is available as a technique for a reduction in the weight of the structural member and the problem of sinkmarks formed on in a resin surface which is unique to resin molding. This technique is disclosed in, for example, U.S. Pat. No. 4,473,665, U.S. Pat. No. 4,360,484, and Japanese Patent Laid-Open Nos. 8-300392 and 10-24436.
In image forming apparatuses such as electrophotographic copying machines, laser beam printers, and facsimile apparatuses, other office equipment, industrial equipment, and the like, vibrations from vibration sources such as rotating members like motors and vibration generating members adversely affect the image forming step, and adverse effects on the information transmission step adversely affect image and information output results.
Information equipment such as a computer incorporates a cooling fan motor as a countermeasure against heat dissipation from electric and electronic elements.
In printers, particularly, laser printers which have remarkably become popular in the recent years owing to an increase in image resolution, the vibrations produced by motors for rotating/driving polygon mirrors greatly influence the resolution of images in apparatuses for transferring image information created by laser scanning to image carriers such as paper through information transfer means such as polygon mirrors, optical lenses, and the like.
The above parts and units such as a motor, optical lens, and laser source are held and incorporated on a mount member in an image forming apparatus.
In the prior art, the mount member is a metal member or a molded product made of a resin material. For example, according to the technique disclosed in Japanese Patent Laid-Open No. 7-232358, to position the respective parts to be held on the mount member, projections are formed on the flat surface of the mount member to specify the positions of the parts, and a resin material used for the projections is compressed to guarantee the dimensional precision of the respective projections.
As the above office equipment and information equipment are widely used as terminals in offices and homes, the personal use of equipment have proliferated. As a consequence, demands have arisen for reductions in weight and cost as well as size.
The mount member is preferably a molded product made of a resin material to fix/place the above motor, laser source, polygon mirror, optical lens, and the like at predetermined positions of the respective parts.
If a molded product made of a resin material is to be used as a mount member, consideration should be given to a reduction in weight. As a technique of reducing the weight of a resin molded member, a molding method of foaming a resin material is disclosed in U.S. Pat. 4,360,484, U.S. Pat. No. 4,473,665 and Japanese Patent Laid-Open No. 8-300392.
In addition, techniques associated with a product molded by foaming a resin material are disclosed in Japanese Patent Laid-Open Nos. 10-24436, 09-48039, and 12-25066.